I have a project where I'd like to mount an Arduino about 200 feet away from the computer, ideally connected by Cat5 (or Cat6) cable. The computer should still be receiving Serial messages from the Arduino.

You need an 'Arduino Serial' board. Scroll down this page about 3/4 of the way http://arduino.cc/en/Main/Boards. Actually if you don't need shield compatibility you could use a Modern Device Bare Bones Board or an Adafruit DC Boarduino. The solution in reply #1 requires more than one USB <--> serial conversion.